A permanent magnet material is one the crucial electrical and electronic materials employed in a wide range of technical fields from domestic electrical appliances to automotive vehicles, communication equipment and peripheral or terminal devices of electronic computers.
In keeping up with the recent demand for high performance and small size in electrical and electronic appliances, high performance is also demanded of permanent magnets. Although the rear earth-cobalt magnet has been conventionally known as a kind of permanent magnet capable of meeting such demand, such rare earth cobalt magnets are required as much as 50 to 60 wt % of cobalt and a large amount of Sm, which is contained in only minor amounts in rare earth ores and is expensive.
In our recent investigations, a ternary compound containing from, boron, and rare earth elements R as essential elements has been found, in which Sm and Co, scarce in natural resources and hence expensive, are not contained as essential elements; light rare earth elements; such as Nd and/or Pr, contained relatively abundantly in rare earth ores, are used predominantly as the rare earth elements and in which superior uniaxial magnetic anisotropy and magnetic properties are displayed, have been realized through the use of iron and boron. Based on this finding, an Fe-B-R sintered magnet showing magnetic anisotropy and high permanent magnetic properties has been proposed, which exhibits a maximum energy product far exceeding that of the conventional rare earth cobalt magnet (Japanese Patent Kokoku Publication No. 34242/1986).
On the other hand, the permanent magnets are subjected to an increasing extent to more and more hostile environments, such as increased self-demagnetizing field resulting from the decreased magnet thickness, strong reverse magnetic fields applied from coils or other magnets or high temperatures resulting from increased operating speeds or increased loads applied to devices or apparatus making use of the magnets.
It has been known that the Fe-B-R magnetically anisotropic sintered magnet containing Nd and/or Pr as the rare earth elements is not affected by slight changes in the composition or the method of production and has a substantially constant temperature coefficient of the coercivity iHc about equal to 0.6%/.degree.C.
Hence, a still higher coercivity is required of the permanent magnet to be employed in such hostile environments.
The assignee of the present application has also proposed an Fe-B-R permanent magnet in which heavy rare earth elements such as Dy and/or Tb are used as a part of R to meet the demand for high coercivity (Japanese Patent Kokai Publication No. 62 32306/1985).
The above mentioned sintered magnet, which exhibits a markedly high coercivity without having a reduced maximum energy product, may also be obtained if the small or trace amounts of impurities contained in industrial level starting materials, such as Al, Si, Cu, Cr, Ni, Mn or Zn, are adjusted, and the starting material so adjusted is subjected to predetermined heat treatment (Japanese Patent Kokai Publication No. 220803/1989).